Measurement while drilling (“MWD”) tools are generally used during drilling for taking directional surveys in real time. For a directional driller to steer the well towards a target zone, the driller must know where the well is going, and the effects of his or her steering efforts. MWD tools often incorporate electrical insulators in the drillstring to transmit data. The tool generates an altered voltage difference between the top part (i.e., the main drillstring, above the electrical insulator), and the bottom part (i.e., the drill bit, and other tools located below the electrical insulator). At the surface, a wire is attached to the wellhead, which makes contact with the drillstring. A second wire is attached to a rod driven into the ground some distance away. The wellhead and the ground rod form two electrodes of a dipole antenna. The voltage difference received between the two electrodes is received by a computer and decoded.
Electrical insulators (e.g., electrical gaps) may be used for both downhole-to-surface communication (i.e., the signal is transmitted to the surface) and downhole-to-downhole communication (i.e., the signal is transmitted to a downhole location). In certain applications, two or more electrical insulators may be employed to accomplish both downhole-to-surface and downhole-to-downhole communication simultaneously. In this case, the existence of one electrical insulator may adversely affects the perform lance of the other electrical insulators.
For an electrical insulator to work properly, the drillstring and the underground formation must form a conducting path through drilling mud so that an electrical current can be driven across the electrical insulator. This generally is not a concern for wellbores drilled with salty or conducting muds because the muds adequately conduct electrical current. However, conducting an electrical current through oil-based or other non-conducting muds may be more difficult. For a BHA employing a single electrical insulator, the drill bit often provides a viable contact point between the drillstring and the formation through which electrical current may flow from the electrical insulator through the drillstring to the formation, and return to the drillstring on the other side of the electrical insulator.
But, for a BHA employing two or more electrical insulators, lower electrical insulators (closer to the bit) may break or impede the path of current flowing from the upper electrical insulator that relies on the drill bit as the contact point. What is needed then is an apparatus and method having multiple electrical insulators that may be operated simultaneously without adversely affecting others' operation.